If you were to compress all of the earth's history into one year, primates would appear about 5 days before the new year, and Homo Sapiens (Man) would appear about 2 hours before the new year. How did mammals become simple primates and then become Homo Sapiens?
Animals adapt by producing anatomical, physiological, and behavioral traits that promote survival and reproduction. Adaptations evolve over time in response to interactions with other organisms and with the physical environment. Animals that adapt better, survive better, and leave more offspring with a copy of their genes than others in the population.
Over 55 million years ago, during the Eocene epoch, the first appearance of primates coincided with extremely rapid global warming. The expansion of humid forests, due to abundant rainfall, became widespread in regions with warm, constant temperatures. Primates during this period weighing less than 1 kilogram ate fruits and insects. Because these primates were aboreal (tree dwellers) many changes occurred. Their hands and feet adapted for grasping with an opposable thumb that could move opposite the other fingers. Their claws changed to fingernails, and their forearms had 2 separate bones that could rotate and allow the palm to face up or down. An upright posture freed their hands for manipulating objects. Because jumping and leaping became their main form of locomotion their eyes moved closer together and faced forward providing 3 dimensional vision and depth of field. This allowed them to forage for fruits, insects, and tree gums. Most importantly, their cerebral cortex grew larger, which enabled them to learn. Fossils of these squirrel-like primates were found in North America, Europe, and Asia.
At the end of the Eocene epoch increasing separation of continents, changes in atmospheric and oceanic circulation, and major mountain-building events caused extreme changes in global climate. This resulted in less stable environments with seasonal temperatures and rainfall changes. This uneven climate resulted in the loss of many primates that couldn't find food.
34 million years ago, the "terminal Eocene event," an ecological event that caused global temperatures to plummet, resulted in the extinction of most primates except for tarsiers, lemurs and lorises who survived in the lowland forests of Asia and Africa.
20 million years ago during the Miocene period early hominoids (apes) evolved. These animals mostly ate fruit and insects and became excellent tree climbers. They had longer gestation and maturation periods. The amount of parental care and mutual grooming increased. But, by the end of this period the climate became drier and the apes had trouble finding high quality foods such as fruit. With diminishing rainforests many of the apes were forced to live in more open vulnerable environments. This caused another mass extinction.
Finally, 6-7 million years ago the monkeys succeeded where the apes failed by establishing a pattern of somewhat faster life histories, an ability to survive on low-quality diets and adapting to quadrupedal movement.
4.4 million years ago in Africa the Hominoids evolved, including orangutans, gibbons, gorillas, chimpanzees, bonobos, and humans. This family of animals evolved differently than the monkeys. They had larger bodies, a shorter and more stable lumbar region, differences in position and musculature of the shoulder, more complex behaviors, more complex brain abilities, and a longer period of infant development and dependency.
The oldest fossil hominids, discovered in 1992 in northwest Africa, are 4.4 million years old. These early humans were intermediate between chimps and later hominids. They stood slightly over a meter tall, weighed about 66 pounds, and appeared to have walked upright on two legs.
The next oldest hominid fossil (3.9 million years old), nicknamed Lucy, had longer arms and shorter legs than ours. Analysis of her skeleton indicates that she spent half her time in trees and half her time on the ground. The larynx was too high in the throat for speaking a complex language.
Homo Erectus appears in the fossil record 1.8 million years ago. These people had larger brains, and were taller and slimmer. They walked on two legs, made stone tools, and probably spoke a complex language.
The evolution of Homo Sapiens is still very sketchy. Many scientists believe he evolved from Homo Erectus. The oldest fossil of Homo Sapiens, the Neanderthal man, is 400,000 years old. The later populations of Homo Sapiens, known as the Cro-Magnons, were found with a variety of tools; cave drawings, and carved objects. The Cro-Magnons of 35,000 years ago were biologically the same as us.